1. Field of the Invention
The present invention relates to a semiconductor laser device comprising an active layer having the strained multiple quantum well structure where well layers and barrier layers having reverse strains respectively are laminated between clad layers.
2. Description of the Related Art
Demand for a semiconductor laser device is increased in a number of the technical fields. In particular, competition to achieve a higher speed in recording and reading operations is getting vehement in the semiconductor laser device for an optical disc. Compound semiconductors of AlGaAs (aluminum gallium arsenide) series are used in an infrared laser element applied to CD, CD-R, or the like, and the infrared laser element has an oscillation wavelength in the band of 780 nm.
At present, in the infrared laser element, the competition for a higher output has been almost completed, and in return, study has been done now actively to achieve a high output of a red laser element applied to DVD (Digital Versatile Disc). In the red laser element, compound semiconductors of AlGaInP (aluminum gallium indium phosphorous) series are used, and the red laser element consisting of these compounds has an oscillation wavelength in the band of 650 nm. Further, a two-wavelength semiconductor laser device in which the infrared laser element and the red laser element are integrated is now being developed.
In recent years, a stable operation is requested at the high-output in red semiconductor laser devices of 300 mW or more, and therefore it is necessary to achieve further improvements in an active layer and an end facet window structure. Examples of the technologies for increasing the output in the red semiconductor laser device of AlGaInP series include the followings.                1) A luminous efficiency is increased through constituting the active layer so as to have the multiple quantum well structure. The multiple quantum well structure means that well layers and barrier layers are laminated.        2) In order to control the catastrophic optical damage (COD) at the end facet due to the high-output operation or surge voltage, the end facet window region for increasing a band gap in the active layer is formed.        3) A carrier density in a p-type dopant layer is increased.        
In the red semiconductor laser device of AlGaInP series, it is known that the luminous efficiency is increased when a compression strain is applied to a quantum well layer in the active layer, and a laser characteristic such as reduction of a threshold current and an operation current can be thereby improved. The compression strain can be generated, for example, by providing layers having a small lattice constant on both sides of the quantum well layer. However, when the applied strain is too large, a crystal defect is generated, which causes an adverse influence on the laser characteristic.
As recited in No. 2833396 of the Japanese Patent Publications, it is known that a tensile strain is applied to a barrier layer (blocking layer, light guide layer) for blocking a light in the well layer so that the sum of products of strain amounts and film thicknesses in an active layer is at least 0. The sum of the products of the strain amounts and the film thicknesses refers to the sum of the products of the strain amounts and the film thickness determined by the respective strain amounts, film thickness and numbers of layers in the well layers and barrier layers.
A further description is given below. Assuming that the well layers and the barrier layers are multilayered in the active layer. The sum of the products of the strain amounts and the film thicknesses in the active layer is defined as ξact provided that the strain amount in the well layer of an ith (i=1, . . . , n) order is εwi and the film thickness thereof is twi, and the strain amount in the barrier layer of a jth (j=1, . . . , m) order is εbj and the film thickness thereof is tbj. Then, the sum of the products of the strain amounts and the film thickness ξact is the total of the sum of the products of the strain amounts and the film thickness in the well layers and the sum of the products of the strain amounts and the film thickness in the barrier layers. The compression strain is handled as plus, while the tensile strain is handled as minus.
The sum of the products of the strain amounts and the film thickness ξact, is defined as ξact=Σ(εwi×twi)+Σ(εbj×tbj). The total sum here becomes the total of the sum of the products of the strain amounts and the film thickness in the well layers and the sum of the products of the strain amounts and the film thickness in the barrier layers over all of the layers (1-n, 1-m).
However, a conventional semiconductor laser device has the following disadvantage, which is described referring to FIGS. 8 and 9. FIG. 8 is a sectional view of a light emitting end facet of the conventional semiconductor laser device.
In vicinity of a ridge region, a stress evaluation was conducted by means of the Raman spectroscopy at a point Q1 in a contact layer 9, a point Q2 in a second p-type clad layer 7, points Q3, Q4 and Q5 in a first p-type clad layer 5, and a point Q6 in n-type clad layer 3, and a result of the evaluation is shown in FIG. 10. It is found out from the result that a very large tensile strain is applied to the points Q1-Q5 in the vicinity of the ridge region.
In the conventional technology, the sum of the products of the strain amounts and the film thickness ξact in an active layer 4 is set to at least 0. Therefore, a difference between the strain amounts in the active layer 4 and the p-type clad layer 5 is increased, and thereby the stress inherent in the element becomes larger. As a result, the catastrophic optical damage (COD) and internal deterioration are generated at the end facet of the laser element in the high-output operation, which causes an adverse influence on reliability.
This is a huge problem in the red laser element of AlGaInP series having such a high output as at least 300 mW, and it is an urgent need to reduce the stress inherent in the element.